When I posted the awesome video of a fire tornado last week, I had only heard rumors of such things. Apparently, they're more common than I thought. Here's another amazing video, and this one is even better: it's longer, and you can see the rotating smoke cloud around the column of fire! This really is a fantastic demonstration of how microscale weather works. Imagine: a fire starts. As the air is heated above the fire, it rises, and the upward motion can be very strong. This leaves a lower pressure spot at the fire, and the air from outside the fire rushes in to fill the gap. The air is very turbulent, and as the inward-moving air from one side hits air coming in from the other, swirls can form. These get amplified by the constant gale of air, and rotation on a larger scale can get started and sustained. The whirlwind gets pumped by the hot air rising, and the next thing you know you've got a full-blown tornado of fire. Watch the video; see how the fire tornado is narrow and well-focused, but the air outside it is rotating more slowly? That's an outcome of a law of physics called the conservation of angular momentum: if you take something that's spinning and shrink it, the rotation rate will increase. You've seen this a bazillion times; figure skaters start spinning, then draw their arms in. Their decreased radius increases their spin, sometimes very dramatically. Water draining out of a bathtub does the same thing, too. We see it in astronomy all the time too: massive stars undergo core collapse at the ends of their lives. The core shrinks so much the spin rate can go up vastly, and we're left with collapsed neutron stars -- mind-numbingly über-dense objects with the mass of the Sun compressed into a ball only a few kilometers across, and they're spinning quite literally a thousand times per second: faster than the blades of a kitchen blender. Angular momentum is a powerful, powerful thing. And it's also beautiful. On scales as titanic as an octillion tons of star matter collapsing to form a weird quantum mechanical fluid, down to an almost supernaturally awe-inspiring column of fire, physics is everywhere, and it's an astonishing thing to watch.
Tip o' the fireman's helmet to Dave Mosher.